Polyolefin foams are useful industrial products because of their excellent heat insulation, cushioning and other properties. These foams have found acceptance over the years in such applications as thermal insulation, flotation, food containers and as raw materials for the fabrication of various shaped articles.
The preparation of non-crosslinked thermoplastic foams by extruding a heat-plastified mixture of a thermoplastic resin and a blowing agent is well known in the art and is described in U.S. Pat. Nos. 2,740,157; 3,067,147; 3,413387; 3,413,388; 3,431,163; 3,431,164; 3,808,300; 3,954,929; 3,966,381; 4,640,933; 4,663,361; 4,694,027; and Canadian Patent No. 451,854, as well as in other literature pertaining to the art.
U.S. Pat. No. 2,450,436 has an early disclosure of a method for the preparation of cellular thermoplastic products. Here, a solid thermoplastic resin, e.g., polystyrene, and a normally gaseous agent, such a methyl chloride, methyl ether, propylene, or butylene are held in a closed vessel under pressure at a temperature below the critical temperature of the normally gaseous agent until a homogeneous mobile gel is obtained. Thereafter, an outlet is opened to permit flow of the gel from the vessel. During the flow of mobile gel from the pressurized vessel into a zone of lower pressure, the resin is swollen by vaporization and expansion of the dissolved volatile substance to form a stable cellular product consisting for the most part of individual closed thin-walled cells.
U.S. Pat. No. 2,515,250 describes a method of forming under pressure a mixture of predetermined proportion of a normally gaseous agent and a thermoplastic resin and storing the mixture by feeding same into a pressurized storage vessel wherein it is maintained at a desired temperature until a homogeneous mobile gel or solution is obtained prior to extrusion and expansion of the resin.
U.S. Pat. No. 3,067,147 discloses a method for the preparation of a cellular mass by incorporating a gas or volatile liquid into a thermoplastic resin, the latter than being heated to a temperature at which it becomes plastic, whereby vapors or gas or volatile liquid expand the softened resin to form a cellular mass.
U.S. Pat. No. 2,387,730 teaches a method of making cellular polyethylene by impregnating a molten polymer with a gas which is soluble therein under pressure. The pressure is then partially released while the temperature is maintained, thus causing the polymer to expand, and the expanded polymer is cooled.
U.S. Pat. No. 3,808,300 discloses a method for the preparation of a closed cellular product of olefin polymers using a mixture of a citric acid salt, a carbonate or bicarbonate as the nucleation agent, and n-butane/isobutane mixture as the blowing agent.
U.S. Pat. Nos. 4,640,933, 4,633,361 and 4,694,027 disclose methods for the preparation of expandable polyolefin compositions using isobutane; or a mixture of isobutane and chlorofluorocarbons and fluorocarbons as the blowing agent; or a mixture of at least 70% isobutane and other hydrocarbons as the blowing agent and a fatty acid amide as a stability control agent.
U.S. Pat. No. 4,308,352 describes a method for producing a polysulfone foam using methylene chloride as a blowing agent.
U.S. Pat. No. 3,646,155 describes a method for producing a crosslinked olefin polymer and copolymer using an unsaturated hydrolyzable silane, but does not mention the uniqueness or advantage of using only silane as a grafting agent to lightly crosslink a polyolefin resin.
U.S. Pat. Nos. 3,098,831 and 3,098,832 describe methods for producing a crosslinked polyethylene foam after it is extruded.
U.S. Pat. No. 3,960,784 discloses a method for making crosslinked synthetic resinous foam bodies from expandable beads or particles.
U.S. Pat. No. 4,142,956 discloses a method for making a crosslinked open cell thermoplastic resin polymer foam which has been expanded with a chemical blowing agent in a batch process.
U.S. Pat. No. 4,163,085 describes a method for producing a continuously extruded sheet which is subsequently crosslinked and expanded once it has left the extrusion die.
U.S. Pat. No. 4,234,531 describes a method for making a crosslinkable extrudate on a cable that is subsequently crosslinked after leaving the extrusion die.
U.S. Pat. No. 4,652,588 discloses a method for making a lightly crosslinked polyethylene/polystyrene blend foam using an organic peroxide.
The heretofore know methods for making cellular bodies from normally solid polyolefin resins have not been entirely satisfactory, since the resins themselves have thus far been possessed of poor melt strength. There is evidence to suggest that resin melt strength is an important characteristic of a resin's ability to be processed into uniform fine celled expanded articles, and that increasing the melt strength of a resin can produce a more desirable product with enhanced properties.
It is thus a primary object of the invention to provide a method for improving the melt strength of normally solid polyolefin resins.
It is a further object of the invention to provide an improved process for producing a polyolefin cellular mass from a normally solid polyolefin resin having an improved melt strength.
Another object of the present invention is to provide a process for making a lightly crosslinked cellular mass from a normally solid thermoplastic polyolefin polymer in a continuous manner.
A further object of the present invention is to provide a process and blowing agent for making a lightly crosslinked cellular mass composed of uniform fine cells from a normally solid thermoplastic polyolefin.
Yet another object of the invention is to provide a process and blowing agent for making a lightly crosslinked cellular polyolefin composed of uniform cells by utilizing a crosslinking agent to lightly crosslink the polyolefin resin prior to or while processing the polyolefin resin in the foam extruder.
Still another object of the present invention is to provide an improved method and apparatus for producing a lightly crosslinked polyolefin foam and articles made therefrom having various geometric configurations and decreased bulk densities.
Another object of the invention is to produce lightly crosslinked polyolefin foams that will have improved melt strength, an increased heat distortion temperature and smaller cell size.
Still another object of the present invention is to produce polyolefin foams having good cushioning qualities, buoyancy and insulation properties.
Yet another object of the present invention is to produce a polyolefin foam that can be laminated to one or more layers of thermoplastic film, paper, foil or other suitable materials to produce a composite that has improved insulating properties.